Point-to-point microwave radio systems are widely used to transmit data between and among two nodes of a communication link. The microwave radio system includes microwave receivers and transmitters at both nodes of the system for transmitting and receiving data. Typically, data is received by a modem which modulates the data onto a baseband or intermediate frequency (IF) signal which is then up-converted to microwave frequencies, amplified and transmitted. The baseband or IF carrier is often provided by a phase-locked loop circuit which is locked to an IF oscillator or frequency synthesizer. In turn, the frequency synthesizer is often provided with a reference oscillator which is also used for the up-conversion of the modulated IF signal to the microwave frequencies.
The microwave signal is received at a remote terminal station through an appropriate microwave radio front end circuit. The receiving radio circuitry receives a locally generated reference signal from a reference oscillator to down-convert the microwave radio signal to a lower intermediate frequency or baseband signal to be demodulated so as to recover the digital data signal encoded onto the carrier signal. The demodulating modem uses a corresponding phase-locked loop circuit synchronized to the incoming baseband signal to recover the data. As in the transmitter circuit, the receiver's phase-locked loop circuit is similarly provided with an intermediate frequency signal from an IF oscillator or frequency synthesizer which is also locked to the reference oscillator.
Minor signal frequency variations between sites are accommodated by the tracking capability of the receiver's phase-locked loop circuitry. The phase-locked loop (PLL) circuit generally includes a phase detector receiving a reference signal together with a sample of the output of the phase-locked loop circuit. The output of the phase detector is provided to a loop filter which, in turn, provides an error signal controlling a voltage controlled oscillator (VCO). The VCO uses the error signal to maintain an output signal having a constant frequency defined by the feedback loop. This is accomplished by sampling the output signal provided by the VCO, dividing the frequency by a programmable counter, and then comparing the frequency divided sample with the reference signal input at the phase detector to provide the error signal.
While the PLL can accommodate and adjust to some frequency variation of the reference signal, its operating range is still limited by certain design criteria. Thus, the PLL must receive an input signal which is within a predetermined capture or pull in range of its free running frequency prior to “locking in,” i.e., operating in a stable mode whereby the error signal provided by the phase detector and loop filter to the voltage control oscillator is within the range of the VCO's operating capability. Once locked, the frequency of the reference signal must be maintained within a hold-in range of frequencies in which the PLL will remain locked to the signal. This range is also known as the lock limit of the PLL. The limited frequency range of the PLL provides a corresponding lock range of the receiver station in which changes of the frequency of the received signal in comparison to the local reference clock frequency can be accommodated. If the difference between the local reference frequency and the received signal becomes too great, the PLL will unlock and the modem will be unable to detect the digital data signal contained in the modulated carrier signal.
Referring to FIG. 6, a radio communications terminal 100 transmits data over a microwave radio frequency link to radio communications terminal 200 which receives, detects and extracts the digital data for processing and/or retransmission to another site.
Communications terminal 100 receives digital data at modulator 112 of modem 110. Modulator 112 further receives an IF carrier signal from phase-locked loop 114 and superimposes thereon the digital data signal to provide a modulated carrier signal to radio circuitry 140. Radio circuitry 140 up-converts, i.e., translates the modulated baseband or IF signal output provided by modulator 112 of modem 110 to a microwave frequency, amplifies and transmits the signal to a receiving terminal. Phase-locked loop 114 of modem 110 receives, and is locked to, an IF frequency signal provided by frequency synthesizer 120 which, in turn, is locked to a reference frequency signal provided by reference oscillator 130.
Reference oscillator 130 also provides a signal to radio circuitry 140 to be used in up-converting the modulated IF signal to a microwave frequency signal, e.g., 38 GHz.
Communications terminal 200 includes radio circuitry 240 amplifying, filtering and down-converting the received microwave frequency signal received from transmitter terminal 100 to provide an IF or baseband output signal to demodulator 212 of modem 210. Demodulator 212 receives the IF or baseband signal and, using a local oscillator signal provided by phase-locked loop 214, recovers the digital signal and provides the same as an output signal corresponding to the input signal of communications terminal 100. Phase-locked loop 214 of modem 210 is locked to an IF signal provided by local oscillator or frequency synthesizer 220 which, in turn, is locked to an output provided by reference oscillator 230. As in the case of the transmitting terminal, reference oscillator 230 is used both for demodulation and for down-conversion between microwave and IF frequencies.
When initially deployed, reference oscillator 130 of transmitting terminal 100 and reference oscillator 230 of receiving terminal 200 are adjusted to provide reference signals having the same nominal frequency or corresponding frequencies. However, the frequencies of the reference oscillators tend to slowly drift over time due to various factors, including component aging. To the extent these oscillators drift at different rates and/or in different directions over time, the nominal frequency of the microwave signal transmitted and the nominal center frequency of the receiving terminal will increasingly differ over time. Within the hold-in range capability of the phase-locked loop in the receiver, such variations can be accommodated by the receiving modem 210. However, as the frequency drift between the terminals becomes more severe, the ability of the receiving PLL to retain a lock on the IF signal provided by radio circuitry 240 will be exceeded and the communications link will fail. It will then be necessary to manually adjust or replace the reference oscillators in the transmitting and receiver terminals 100 and 200, respectively, to bring the system back into frequency alignment. During this time, of course, the radio communications link is inoperative.
Accordingly, a need exists for a communications system which is immune to or can accommodate long term frequency drift of its internal frequency standard reference. A still further need exists for a modem which can retain a locked condition over a wide range of IF input signals without requiring an automatic frequency control circuit to have a disadvantageously wide capture, acquisition, or hold range capability. A still further need exists for a communications system which does not require expensive, highly stable reference frequency standards to operate properly and avoid loss of signal lock.